text: In Light Induced Excited Spin State Trapping (LIESST), the HS-LS transition is triggered by irradiating the sample. At low temperatures it is possible to trap compounds in the HS state - a phenomenon known as the LIESST effect. The compound can be converted back to a LS state by irradiation with a photon of different energy. Irradiation of d-d transitions of the LS metal complex or MLCT absorption bands leads to population of HS states. A good example to illustrate the LIESST effect is the complex [Fe(1-propyltetrazole)](BF4). The sample was irradiated with green light at temperatures below 50 K. By doing this, a spin allowed transition is promoted which is A → T.  However, the T excited state has a very short lifetime, decreasing the probability for the excited state to relax via a double intersystem crossing to reach the T HS state .  Since the HS state is spin forbidden the lifetime for this state is long, therefore it can be trapped at low temperatures.
  Due to the aim to design photoswitchable materials that have higher working temperatures than those reported to date (~80 K), along with long-lifetime photoexcited states, another strategy for SCO called Ligand-Driven Light Induced Spin Change (LD-LISC) has been studied. This method consists of using a ligand that is photosensitive in order to trigger the spin interconversion of the metal ion and exciting this ligand with light. The LD-LISC effect is followed by a structural change of the photoresponsive ligands in contrast to the SCO process where the structures of the ligands are essentially unaffected. The driving force behind the metal ion SCO in this photochemical transformation is cis-trans photoisomerization. The prerequisite for LD-LISC to be observed is that the two complexes formed with the ligand photoisomers, must exhibit different magnetic behaviors as a function of temperature. Upon successive irradiations of the system at two different wavelengths within a temperature range where the metal ion can either be LS or HS, a spin-state interconversion should occur. In order to achieve this, it is convenient to design a metal environment to where at least one of the complexes exhibits a thermally induced SCO. The LD-LISC has been observed in several iron(II) and iron(III) complexes.